menu and toolbar: separators can be focused with mouse click

You can focus a MenuSeparator? and a ToolbarSeparator? and should not be allowed to. This is because these widgets get assigned a tabindex=-1 when added to the menu and toolbar. Did a quick fix for 1.0 to prevent an error (see #4927) but need to create a more complete solution.

The problem can be seen if you focus a toolbar button, then click on a toolbar separator that is not close to the button. The separator gets focus and the previously focused button loses focus indication. If you then press a right arrow key, focus will go to the button that was next to the previously focused button. Which is what we want but is visually odd.

The same problem occurs in the context menu but is not as evident since the menu item which was focused before the separator is clicked still "looks" focused.

Problem deals with code for arrow navigation between buttons / menu items. That code doesn't know that menuSeparator is special and shouldn't be focused. I guess we need some flag on widgets that aren't focusable (or widgets that are focusable).